This invention relates to injection molding and more particularly to a valve pin bushing which provides improved melt feed around each valve pin in a multi-cavity valve gated injection molding system.
It is well known to use a valve pin bushing which provides a seal around a reciprocating valve pin and also has a melt duct which forms part of the melt passage leading to the gate. An early example of this is shown in U.S. Pat. No. 4,026,518 to Gellert entitled "Bushing Seal for Valve-Gated Injection Mold" which issued May 31, 1977. More recently, it has also become well known to use valve pin bushings in multi-cavity injection molding systems. As shown in U.S. Pat. No. 4,433,969 to Gellert entitled "Injection Molding Valve Pin Bushing and Method" which issued Feb. 28, 1984, each bushing is located between the manifold and a respective heated nozzle with a collar portion extending into an opening in the manifold. Each bushing has an outer flanged portion which abuts against the surrounding cavity plate to laterally locate the valve pin bushing, while sufficient clearance is provided through the opening in the manifold to allow for thermal expansion of the manifold. A melt duct extends diagonally through each bushing to join an enlarged portion of the central bore through which the valve pin extends. While this previous arrangement is suitable for many applications, when molding some materials such as white polyvinyl chloride the pattern of flow of the melt from the diagonal melt duct around the valve pin creates discoloured streaks where the flow joins again on the other side. This also delays the elimination of the previous coloured melt on colour changes because of slower melt flow on the opposite side of the valve pin.